This invention relates to an optical member, particularly an optical filter. More particularly, the present invention is concerned with an optical filter consisting essentially of a copolymer which exhibits thermotropic cholesteric liquid crystal properties and comprising specific two kinds of L-glutamates and/or D-glutamates, and suitable for use as an optical filter for a color display and a branching filter for synthesizing or separating multiple optical signals, and a notch filter, a band-pass filter, a circularly polarizing filter and an optical isolator which are used for various applications.
An optical member, particularly an optical filter is an important structural element in the field of optoelectronics such as displays and optical communications, and filters made of various materials and having various functions are used therefor. For example, in the field of a color display, a filter comprising a number of arranged picture elements comprising color filters respectively for R (red), G (green) and B (blue) is used. In this case, since the spectroscopic wavelength ranges respectively for R, G and B filters partially overlap with each other, a notch filter is used for cutting off the wavelength in the overlapped ranges. Although a notch filter made of an acryl resin containing neodymium has conventionally been used, it has disadvantages such as disagreement of the absorption wavelength range with the wavelength range to be cut off, small degree of absorption and difficulty in forming it into a thin sheet. Therefore, the development of a high performance notch filter capable of freely controlling the wavelength ranges to be cut off and being produced and processed with ease has been demanded.
Meanwhile, in the field of optical communications, wavelength multiplex transmission techniques have been developed to increase the transmission capacity per optical fiber and is expected to be applied to a subscriber transmission system. In the wavelength multiplex transmission system, a branching filter for synthesizing or separating multiple optical signals should be necessarily used, and an interference film optical branching filter and a diffraction grating optical branching filter have conventionally been developed therefor. The former is advantageous in that the central wavelength and bandwidth can be freely designed but disadvantageously requires extremely sophisticated techniques in forming an interference film and much work, causing not only difficulty in producing it but also increase in cost with the increase in the wave number to be separated. On the other hand, although the latter has an advantage that a number of waves can be separated using one diffraction grating, it is disadvantageous not only in that the wavelength band in which a high diffraction grating efficiency can be achieved is limited to that near the blaze wavelength, causing difficulty in widening the band, but also in that it requires sophisticated producing techniques and is high in cost. Therefore, also in the field of optical fiber communications, the production of a high precision band-pass filter or notch filter is extremely useful because such band-pass filter or notch filter can be used as a branching filter which is easy to produce, light in weight and low in cost.
Besides the above-mentioned fields, various optical filters such as an optical isolator, a half mirror and a circularly polarizing filter are used in the fields of optoelectronics and optics as well. These optical filters have serious disadvantages such as difficulties in producing them and maintenance and high cost and, sometimes, insufficient performance, because they are produced by combining a lens, a prism, a diffraction grating, etc. which have been generally produced by precision working of glass, metals or inorganic materials with a sophisticated technique.
Meanwhile, attempts have been made to apply specific optical effects of a cholesteric liquid crystal to an optical member. When the pitch of the helical structure is p and the average refractive index n, if the incident light is perpendicular to the sheet surface, the cholesteric liquid crystal has a wavelength, .lambda., corresponding to .lambda.=p.n, and selectively reflects a right-handed circularly polarized light or a left-handed circularly polarized light according to the direction of the helicalaxis. Therefore, the cholesteric liquid crystal can be used as a notch filter, a band-pass filter, a circularly polarizing filter, etc. by skillfully taking advantage of the above-mentioned properties. For example, Japanese Patent Laid-Open No. 139506/1981 discloses a visible light reflecting polymer composite comprising a cholesteric liquid crystal having a helical pitch capable of selectively reflecting light belonging to a visible region and immobilized by an amorphous polymer. The visible light reflecting polymer composite is prepared by a method which comprises forming a cholesteric liquid crystal from a polyglutamate (L- or D-isomer), which is known as having lyotropic cholesteric liquid crystal properties, using a photopolymerizable unsaturated monomer as a solvent and exposing the mixture to light rays to polymerize the unsaturated monomer and to immobilize the cholesteric structure by the resulting polymer. In the above-mentioned method, since the liquid crystal used is limited to a polymer which exhibits lyotropic cholesteric liquid crystal properties, and it has a structure immobilized by polymerizing an unsaturated monomer which has been used as a solvent, not only difficulties are encountered in finding out conditions of preparation under which the pitch of the cholesteric liquid crystal can freely be varied but also it is impossible to vary the pitch of the cholesteric liquid crystal, once it has been prepared, after immobilization thereof. Japanese Patent Laid-Open No. 191203/1985 discloses a method of preparing a notch filter which comprises forming lyotropic cholesteric liquid crystals from poly-L-glutamic acid .gamma.-butyl ester and poly-D-glutamic acid .gamma.-butyl ester each using triethylene glycol dimethacrylate as a solvent with the same technique as mentioned above, immobilizing the liquid crystals by photopolymerization and laminating the resulting two films. However, this method has the same disadvantages as that mentioned above with regard to Japanese Patent Laid-Open No. 139506/1981 because the two methods use the same techniques.
The present inventors have previously found that a copolymer of a specific two kinds of glutamates exhibits thermotropic cholesteric liquid crystal properties. This copolymer has various advantages as follows. Since the copolymer is thermotropic, there is no need for using any solvent in developing a cholesteric structure as opposed to the above-mentioned cases where the polymer is lyotropic. Further, the cholesteric structure can be immobilized simply by rapidly cooling a heated copolymer, which leads to an extremely easy production of an immobilized liquid crystal. Moreover, the pitch, i.e. selective reflection wavelength, can freely be varied with great advantages by heating the immobilized cholesteric structure which has once been prepared at a temperature different from that used in preparing it. Since these properties are suitable for preparation of various optical filters which require a precise control of transmitting wavelength and cutting wavelength, it is expected that the application to these optical filters will produce various favorable effects with great advantages.